Checking isinstance(obj,Iterable) detects classes that are registered
as Iterable or that have an __iter__() method, but it does
not detect classes that iterate with the __getitem__() method.
The only reliable way to determine whether an object is iterable
is to call iter(obj).

Implementation note: Some of the mixin methods, such as
__iter__(), __reversed__() and index(), make
repeated calls to the underlying __getitem__() method.
Consequently, if __getitem__() is implemented with constant
access speed, the mixin methods will have linear performance;
however, if the underlying method is linear (as it would be with a
linked list), the mixins will have quadratic performance and will
likely need to be overridden.

Changed in version 3.5: The index() method added support for stop and start
arguments.

ABC for asynchronous generator classes that implement the protocol
defined in PEP 525 and PEP 492.

New in version 3.6.

These ABCs allow us to ask classes or instances if they provide
particular functionality, for example:

size=Noneifisinstance(myvar,collections.abc.Sized):size=len(myvar)

Several of the ABCs are also useful as mixins that make it easier to develop
classes supporting container APIs. For example, to write a class supporting
the full Set API, it is only necessary to supply the three underlying
abstract methods: __contains__(), __iter__(), and __len__().
The ABC supplies the remaining methods such as __and__() and
isdisjoint():

classListBasedSet(collections.abc.Set):''' Alternate set implementation favoring space over speed and not requiring the set elements to be hashable. '''def__init__(self,iterable):self.elements=lst=[]forvalueiniterable:ifvaluenotinlst:lst.append(value)def__iter__(self):returniter(self.elements)def__contains__(self,value):returnvalueinself.elementsdef__len__(self):returnlen(self.elements)s1=ListBasedSet('abcdef')s2=ListBasedSet('defghi')overlap=s1&s2# The __and__() method is supported automatically

Since some set operations create new sets, the default mixin methods need
a way to create new instances from an iterable. The class constructor is
assumed to have a signature in the form ClassName(iterable).
That assumption is factored-out to an internal classmethod called
_from_iterable() which calls cls(iterable) to produce a new set.
If the Set mixin is being used in a class with a different
constructor signature, you will need to override _from_iterable()
with a classmethod that can construct new instances from
an iterable argument.

To override the comparisons (presumably for speed, as the
semantics are fixed), redefine __le__() and __ge__(),
then the other operations will automatically follow suit.

The Set mixin provides a _hash() method to compute a hash value
for the set; however, __hash__() is not defined because not all sets
are hashable or immutable. To add set hashability using mixins,
inherit from both Set() and Hashable(), then define
__hash__=Set._hash.